There is a growing market for flat panel displays in a wide variety of uses. Small flat panel displays are used in, for example, mobile phones and digital cameras. Larger panels are used as screens in laptop computers. Still larger panels, having a diagonal dimension of for example 13 or 17 inches, are becoming very popular as monitors for desktop personal computers. Still larger panels are being used increasingly in television receivers.
In its essence, a flat panel display comprises an array of pixels on a substrate. The pixels may comprise liquid crystals, or organic light emitting diodes, or OLEDs. In both cases a picture is formed by selectively activating pixels within the grid. The picture quality and resolution is determined to a large extent by the level of sophistication of the system for driving the pixels.
The segment drive method is designed to apply a voltage to discrete groups of pixels at the same time. This method is suitable for simple displays such as those in calculators, but are unsuitable for more sophisticated displays because of their relatively poor resolution.
For higher resolution displays a form of matrix drive method to is required. Two types of drive method are used for matrix displays. In the static, or direct, drive method each pixel is individually wired to a driver. This is a simple driving method, but as the number of pixels is increased the wiring becomes very complex. An alternative method is the multiplex method, in which the pixels are arranged and wired in a matrix format. There are two types of matrix format, passive matrix and active matrix.
In passive matrix displays there are no switching devices, and each pixel is activated more than one frame time. The effective voltage applied to the pixel must average the signal pulses over several frame times, which results in a slow response time and a reduction of the maximum contrast ratio. The addressing of a passive matrix display also results in cross-talk that produces blurred images, because non-selected pixels are driven through a secondary signal voltage path.
In active matrix displays a switching device and a combined storage capacitor can be integrated at each cross point of the electrodes, such that each pixel has at least one switching device. Active matrix displays have no inherent limitation in the number of scan lines, and present fewer cross-talk issues. Most active matrix displays use switching devices that are transistors made of deposited thin films, known as thin film transistors or TFTs.
An essential part of any TFT is a thin layer of semi-conducting material. A suitable semiconducting material is doped silicon. If doped silicon is used, it is usually in the form of amorphous silicon (a-Si). Amorphous silicon TFTs can be made in large area fabrication at a relatively low temperature (300 to 400 degrees C.).
Poly-crystalline silicon (p-Si) or microcrystalline silicon (mc-Si) is superior to amorphous silicon in that it has an electron mobility that is 1 or 2 orders of magnitude greater than that of amorphous silicon. However, polycrystalline and microcrystalline silicon are costly to make and especially difficult to fabricate when manufacturing large area displays.
The creation of a TFT requires a significant number of individual process steps, each of which needs to be carried out in a vacuum or a controlled atmosphere in order to avoid contamination. Even small amounts of contamination may seriously hamper or even destroy the functionality of the TFT. The creation of a flat panel display requires additional steps of depositing materials, such as the display materials themselves and encapsulation layers protecting the TFTs and display materials from ambient influences.
The current practice is to conduct these individual process steps in separate, dedicated machines. This requires that all individual machines necessary for manufacturing a flat panel display be assembled in a clean room, and that the substrates be moved from one machine to the next. Manufacturing logistics often require that substrates be stored for some length of time in-between process steps. To avoid contamination, this storage also needs to take place in a clean room environment.
Manufacturers of flat panel displays have sought to overcome the inherent cost of these manufacturing processes by using large substrate panels upon which several flat panel displays can be manufactured at one time. Although this approach allows for a certain economy of scale, it requires the handling of ever larger substrate panels with the accompanying cost of requiring increasingly costly equipment for carrying out the steps of the manufacturing process, and for conveying the substrate panels from one process step to the next.
It is, therefore, and object of the present invention to provide an in-line process for the manufacture of thin film transistors on a substrate. It is a further object of the present invention to integrate the in-line process for manufacturing thin film transistors into an in-line process for manufacturing flat panel displays.